GNU C/C++/Objective-C 4.2.1 Compiler User Guide

10.5 Fixed Header Files

GCC needs to install corrected versions of some system header files.
This is because most target systems have some header files that won't
work with GCC unless they are changed. Some have bugs, some are
incompatible with ISO C, and some depend on special features of other
compilers.

Installing GCC automatically creates and installs the fixed header
files, by running a program called fixincludes. Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.

If you update the system's header files, such as by installing a new
system version, the fixed header files of GCC are not automatically
updated. They can be updated using the mkheaders script
installed in
libexecdir/gcc/target/version/install-tools/.

On some systems, header file directories contain
machine-specific symbolic links in certain places. This makes it
possible to share most of the header files among hosts running the
same version of the system on different machine models.

The programs that fix the header files do not understand this special
way of using symbolic links; therefore, the directory of fixed header
files is good only for the machine model used to build it.

It is possible to make separate sets of fixed header files for the
different machine models, and arrange a structure of symbolic links so
as to use the proper set, but you'll have to do this by hand.